“Just stick ’em in the sunshine, buddy,” a coarse voice jabbed at me from the window of the second floor. Years later, I recall those words and chuckle. It must have been quite amusing for that carpenter to watch me methodically roam the grounds in search of the optimal site for the photovoltaic array.
Installing modules in a sunny, shade-free spot and pointing them toward the sun could be considered common sense to many, but properly orienting and tilting your array for optimal performance is not as intuitive. A PV array’s output is proportional to the direct sunlight it receives. Even though PV modules produce some energy in a shady location or without ideal orientation, system costs are high enough that most will want to maximize energy yield.
Sun-worshippers working on their tans will orient their bodies to the sun for the most “exposure,” changing their body position to coordinate with the sun’s changing position. The lesson for PV arrays is that orienting directly toward the sun results in capturing the most energy—maximum power comes when they’re perpendicular to the sun’s rays.
Trace the sun through the sky and you’ll notice two things changing. Throughout the day, its elevation (angle to the horizon) changes, rising to reach its maximum height at “solar” noon. And its sweep from east to west also changes as the day progresses, depending on the latitude. At high northern latitudes in the summer months, for example, it may rise in the northeast and set in the northwest. The angle it sweeps is known as the “azimuth” angle. Both the elevation and azimuth angle change with the seasons.
Most PV systems are mounted on racks in a fixed position, usually on a rooftop, and don’t have the capability to follow (track) the sun throughout the day. Considering this, the best orientation in the Northern Hemisphere is usually due south. However, there are site-specific exceptions to the rule. For example, if your site consistently has fog or clouds at one end of the day, then favoring an orientation away from these times may result in better production from your array.
Before you use your compass to site an array, you must correct for your site’s declination error. In the Northern Hemisphere, a compass needle aligns itself along the magnetic north-south line. A PV array should be oriented to “true” or “solar” south (“geographic” south), so you’ll need to account for magnetic declination—the angular difference between true and magnetic north. The main cause for this discrepancy is the Earth’s non-uniform, conductive, fluid outer core that consists mainly of iron and nickel. This layer pushes your compass needle off of true north. Depending upon your location on the planet, the “push” varies in strength and direction. (See HP120, “Finding True South the Easy Way.”)
The National Oceanic and Atmospheric Administration (NOAA) maintains a Web site that gives up-to-date magnetic declination values for the United States (see Access). Input your zip code in the form, and it provides your current magnetic declination. For example, in Port Angeles, Washington, the angle of magnetic declination is 17.5° east.
Accounting for declination is fairly straightforward using a compass: If your magnetic declination is east, then you subtract your declination from the magnetic north (0°; 360°) and magnetic south (180°) readings to get your “true” directional readings. Using our Port Angeles declination of 17.5°E, true north and south align with 342.5° (360° – 17.5°) and 162.5° (180° – 17.5°). To correct for a west declination, add the declination to 0° (360°) and 180° to get your true directions.
With solar south determined, the next step is to find the optimum tilt for the array. Because a PV module performs best when its surface is perpendicular to the sun’s rays, considering the sun’s elevation in the sky is important.
In the Northern Hemisphere, the sun rises to its greatest height at noon on the summer solstice (about June 21). It sinks to its lowest angle at noon on the winter solstice (about December 21). These elevations vary depending on your location’s latitude. In Seattle, Washington, which sits at 47°36’N latitude, the solar-noon elevations span from 19° in winter to 66° in summer. In Miami, Florida, which has a latitude of 26°N, these angles span from 41° to 88°.
Typically, fixed (non-adjustable) PV arrays should be tilted toward the sun’s “average” elevation—equal to the latitude of the array’s location—to capture the most year-round solar energy. For example, I live at about 36°N latitude. Generally, to produce the most energy over an entire year at my location, a fixed array should be oriented to true south and set at a tilt of 36° from the ground.
Again, local weather conditions may require special considerations. Very cloudy winters coupled with very clear summers will push the ideal PV tilt to shallower angles. Where snowy winters predominate, a steeper angle may be best for the modules to shed snow. Be sure to pay attention to your local weather patterns.
Another option is an adjustable mount, which can be ground-, pole-, or roof-mounted. The standard adjustment is to add 15° to the location’s latitude for the winter tilt angle and subtract 15° from the latitude to arrive at the summer tilt. Adjusting twice a year on the spring and fall equinoxes can increase production by about 5%—with quarterly adjustments, a little more can be gained. In the winter, especially, these few extra kilowatt-hours can be crucial to an off-grid home, but most on-grid system owners choose the simplicity of a fixed array.
Most homes are not perfectly oriented for a roof-mounted PV array, but that shouldn’t dissuade your plans to have solar-generated electricity.
For roofs that do not face south, one option is to rotate the PV rack to orient the modules south. This maximizes energy production—but by how much and at what visual cost? An oddly racked PV array is unacceptable to some people for aesthetic reasons, and may even be disallowed in some communities with homeowner’s associations or architectural review boards. While an east- or west-facing array usually won’t produce as much energy as a south-facing one, the losses are often less than expected. An array oriented 30° off of true south will typically suffer less than a 5% reduction in production compared to one oriented to true south.
While the best annual performance generally comes from a PV array that’s mounted at a tilt equal to your location’s latitude, your roof pitch may not match this parameter. Don’t worry—an array tilted 15° off from your latitude will still produce 95% of the energy from an array tilted at latitude. For aesthetics and to reduce wind loads, the vast majority of residential roof-mounted PV arrays are installed parallel to the roof plane.
Compare the energy production of the two different racking scenarios for a 1 kW PV array in the “Array Tilt & Azimuth: Effects on Efficiency” table on the opposite page. In this scenario, optimal orientation would translate into a more costly and awkward mounting system that will squeeze out only a few more kilowatt-hours per year. Again, slightly increasing the array size with the flush-mount option could make up for these minor losses.
Many contemporary homes sport complex roof designs with multiple, pitched roof sections of varying sizes, dimensions, and orientations that can present a complex challenge to PV system designers.
A good design practice is to plan for using only about 80% of the area on any particular roof section, leaving enough space around the array for safe access. You may have two adequately sized roof sections with acceptable exposure that face southeast and southwest, and you need both of these sections to accommodate the size of the planned PV array. Ensure adequate solar exposure by checking for shadows from the rooflines, chimney, or HVAC equipment with your solar siting tool of choice (see Access).
Multiple Orientations & MPPT. Most residential grid-tied inverters have only one maximum power point tracker (MPPT) incorporated into their design. A MPPT constantly sweeps the array output to fine-tune its power algorithm, resulting in the maximum power output from the PV array. Modules experiencing the same environmental conditions, i.e., mounted on the same roof surface, can use a single MPPT. But PV modules on separate roof surfaces, whether at different orientations or tilts, experience slightly different environmental conditions and, therefore, different maximum power points. The best performance would come from maximum power point tracking for each PV orientation, either by using multiple inverters or a single inverter with multiple MPPT capability.
Time-of-Use Metering. Typically, utility companies use a constant rate to charge a customer for each kWh consumed: A kWh consumed at 3 p.m. costs the same as a kWh consumed at 8 a.m. However, a growing number of utilities are offering time-of-use (TOU) metering systems that charge different rates based on when electricity is used. Rates are highest during peak hours (typically weekday afternoons) and lowest during nonpeak hours (early mornings and weekends). With net metering under TOU, any energy put back into the grid during peak times is counted at the higher rate—up to three times the value of off-peak electricity.
If you can optimize your grid-tied PV array performance to generate the most electricity during peak rate periods, you’ll reap a bigger payoff and help out the utility grid when it needs it most. Examining various orientation scenarios within the context of a net-metering grid interconnection (PV electricity and utility electricity is valued at the same rate) can be a valuable exercise before you site your array. Utility peak times and rates typically vary with the seasons. Before you make a decision about orienting your system to favor TOU metering, check your utility’s rates carefully.
Orientation, array tilt, seasonal adjustments, and array siting can all affect the bottom line. Proper planning and smart design will help you get the most out of your PV system and improve your rate of return.
David Del Vecchio, a mechanical engineer from Georgia Tech, began designing and installing PV systems in 1998 in North Carolina. He’s a NABCEP-certified PV installer and owns Solar Seed, a consult/design business. David also teaches PV courses for Solar Energy International, the North Carolina Solar Center, and Central Carolina Community College.
NOAA • www.ngdc.noaa.gov/geomagmodels/Declination.jsp • Magnetic declination calculator
http://solardat.uoregon.edu/index.html • Sun-path diagram calculator
http://www.srrb.noaa.gov/highlights/sunrise/azel.html • Solar position calculator
PVWatts • pvwatts.nrel.gov • Solar energy calculator
Solar Siting Tools:
Solar Pathfinder • www.solarpathfinder.com
Solmetric • www.solmetric.com • SunEye
Wiley Electronics LLC • www.we-llc.com • ASSET